1. Field of the Invention
The present invention relates to a blank for magnetic hard disk produced by blanking an aluminium- or aluminium alloy plate in the form of a disk, and a method for producing the same. More specifically, the present invention relates to a blank for magnetic hard disk and a method for producing the same, characterized in that even after annealing such blank stacked on each other, the blank can be peeled off from each other.
2. Description of the Related Art
The magnetic hard disk materials to be used as a recording medium for computer systems and the like should be demanded to have the following properties;
1. the materials should be of light weight; PA0 2. the materials should be non-magnetic; PA0 3. the materials should be rigid; PA0 4. the materials can readily procure excellent surface precision by precise processing and grinding; and PA0 5. the materials should be inexpensive. PA0 1. finishing a product of a given dimension; and PA0 2. for subsequent plating process and polishing process, removing the oxide film layer on the blank surface and decreasing the surface roughness of the blank prior to plating process. The surface roughness of the blank is about 30 nm after the substrate processing.
Therefore, aluminium alloy plates are generally used as magnetic hard disk materials. Aluminium alloy plates of 5,000 series, such as 5086 alloy, in particular, are excellent in terms of the properties described above. Therefore, such aluminium alloy plates are used, in the forms of coating-type media or thin-film media, as aluminium materials for hard disk drive (HDD) as a recording medium for computer systems.
Such aluminium disks are generally produced as follows. Producing firstly an aluminium alloy ingot, the ingot is then subjected to soaking process and hot rolling prior to cold rolling, to prepare an aluminium alloy plate at a given plate thickness. The resulting aluminium alloy plate is blanked into a disk form, which is defined as so-called blank. The blank thus prepared is stacked together for pressure loading so as to remove the distortion and for subsequent annealing while the stacked blanks are kept at a temperature above 300.degree. C. (such annealing is referred to as `disk annealing` hereinafter). The individual blanks after such disk annealing adhere to each other because the blanks are in contact to each other, at such a high temperature under a high pressure for a long period, which causes Al and Mg atoms in one blank to be transferred into an adjacent blank. Hence, the individual blanks should necessarily be peeled off from each other after annealing.
After the termination of peeling, such individual blanks should be subjected to substrate processing. The term `substrate processing` means a process of grinding the surface of a blank by means of a grindstone of about #3000. The processing is for the following purposes;
By processing the substrate-processed blank with Ni--P plating of a thickness of 20 .mu.m or less, intensity and hardness can be imparted to the disk to prevent the occurrence of data error because of disk damage. Furthermore, the plating film should be polished so as to remove so-called plating defects developed in the Ni--P plating and to prepare the Ni--P plating film into a smooth film. The process makes a magnetic head at low flotation, to yield highly dense recording.
Following the cost down of computer systems in recent years, the cost down of the substrate processing has been demanded. As the method for decreasing the cost of the substrate processing, a method has been known, comprising preliminarily removing the surface oxide film of an aluminium alloy plate prior to grinding, thereby improving the grinding rate. For aluminium disk production on the basis of the method described above, investigations have been made of a method comprising melting and removing the surface oxide film of a blank with nitric acid or phosphoric acid prior to blanking of an aluminium alloy plate into a disk form and subjecting the resulting blank to substrate processing.
During the process of continuously melting and removing the surface oxide film of an aluminium alloy plate with nitric acid or phosphoric acid while winding off the aluminum alloy plate in a coil, however, the blank should adhere to each other because the blank is in contact, through the clean aluminium surface with no oxide film, to each other which causes the transfer of Al and Mg atoms between such blanks when the blank is stacked together at a high temperature and a high pressure for disk annealing. Hence, the individual blanks adhere to each other more tightly, so a higher peeling strength is required than those by conventional processes, when these blanks are to be peeled off from each other after the completion of disk annealing. Therefore, the individual blanks are peeled off from each other with much more difficulty. Consequently, the productivity of aluminium disk is down, involving potential damage of disk surface during peeling.
During blanking process, additionally, the aluminium alloy plate with the surface hard oxide film preliminarily removed with nitric acid or phosphoric acid is disadvantageously more readily damaged of the surface thereof than aluminium alloy plates with the surface oxide film never removed.
So as to enhance the peelability of a disk after disk annealing, it is effective that an appropriate oxide film is present, as an inclusion for preventing the transfer of Al and Mg atoms, on an aluminium material, and a method therefor is proposed as is found in Japanese Patent Laid-open No. Hei 6-84170. Adversely, however, the presence of such oxide film makes more difficult the grinding process of a blank material into a substrate, involving the decrease of the productivity, disadvantageously. So as to prevent this, a blank after disk annealing and peeling should necessarily be washed with nitric acid or phosphoric acid so as to remove the oxide film prior to surface grinding. Then, washing process should be introduced. Additionally in this case, individual peeled-off blanks should be washed at a minimum unit (one blank at the lower limit) each time. In any way, the productivity will be decreased from the respect of the overall substrate production process, which is not necessarily effective.
In such problematic circumstances, the present invention has been attained. The object of the present invention is to provide a blank for magnetic hard disk and a method for producing the same, characterized in that even after removing the oxide film of the blank so as to improve the grinding rate, the adhesion or attachment of the blank to each other can be prevented during disk annealing, thereby improving the productivity of magnetic hard disk and that the damage of the surface of the blank can be prevented during the blanking process.